One of the common fuel metering systems for motor vehicles is known as the common rail system. In this system, fuel is delivered by a first fuel pump from a fuel reservoir to a first pressure area. Starting from this first pressure area, the fuel is transported by a high pressure pump into a common rail which is under very high pressure. In the case of a diesel common rail system, pressures for example may reach 2000 bar, and in the case of a gasoline common rail system, pressures may reach up to 150 bar. The fuel is injected by a fuel injector at predefinable points in time from the common rail directly into a combustion chamber of the combustion engine.
Normally there is a pressure regulating element, for example a pressure control valve or the like, located in the high pressure area, usually directly on the common rail. This pressure regulating element is actuatable by a control unit, usually the engine control unit, in such a way that various pressures are attainable in the common rail. In this case, the various pressures are matched to the various operating situations, in which different pressures make sense for optimizing the combustion process.
A special problem of a common rail system arises in the starting phase, since at the moment the engine starts, the pressure in the common rail in an extreme case corresponds merely to an admission pressure. It is thus necessary for optimal combustion to elevate the pressure in the common rail to normal operating pressure as quickly as possible. To this end, for example, the control unit specifies that a maximum rail pressure be set by the pressure regulating element. During the phase in which the rail pressure is rising to its normal level, there is naturally a very great change in pressure in the rail over time, which makes it difficult to meter fuel as intended. Due to the transient pressure conditions in the common rail, it is very difficult for the control unit to calculate an injection time that corresponds to the fuel injection quantity actually desired. This also sometimes results from the fact that a computation of the fuel injection time is performed first, and only afterward is it possible to actuate the fuel injector accordingly. Thus there is a time difference between the computation of the injection and the actual injection itself. Within this period there is a possibility, that because of the transient pressure conditions in the common rail, a fuel pressure that is different from that intended by the control unit when calculating the injection time will develop.